Measurement of Skin Friction at Low Subsonic Speeds by the Razor-Blade Technique

The technique consists of forming a surface pitot-tube by placing a small segment of razor blade on the surface with its tapered cutting edge above a static-pressure hole. The effects of limited changes in the razor-blade geometry on the measured pressure have been determined in a two-dimensional turbulent boundary layer and a calibration curve for a particular standardised geometry deduced. The variation of pressure due to yawing the blade segments in three-dimensional boundary layers, similar to those likely to occur on aerodynamic models, is found to be independent of the nature of the boundary layer. From this a method of using razor-blade segments to measure skin friction in three-dimensional boundary layers is proposed, which does not necessitate prior knowledge of the surface-flow direction.